This invention relates to communications systems and more specifically to communications systems including multiple independent communications modules which may be referenced to a common external signal.
Modern communication systems for use on vehicles such as aircraft frequently include a variety of communications equipment and a number of independently operating communications modules for such purposes as voice communication, data links, and satellite feeds. Since vehicular platforms are of limited size, these modules are often in relatively close physical proximity to one another to the extent that they may electrically interfere with each others operation. In order to help control this problem, the individual communications modules comprising the system may be referenced to a single external signal which functions as a source for the reference base used by each of the modules, thereby phase synchronizing their operations. Referencing the modules within the system to a single external signal does not totally eliminate intra-system interference but does assist in regulating such interference by giving the associated distortion a more regular and predictable pattern which is less disruptive to communications functions and more easily controlled.
The external reference signal supplied in common between the modules is not suitable in itself for use as a reference base by the individual modules since it is subject to substantial amounts of distortion and may be at a different frequency than the required base signal. Therefore, past systems in which individual equipment modules have been referenced to a single external signal have included narrow band pass filters and frequency dividers/multipliers for carefully filtering and accurately translating the external reference signal to form a suitable reference base for each individual equipment unit. Since the reference base must be a very stable, accurate and clean signal, it has been expensive and difficult to provide the circuitry necessary to perform the required filtering and translating functions. Furthermore, the processing circuits previously provided have had limited flexibility and have required manual switching at each module to select between external and internally generated reference signals and to select for reference signals having different frequencies.
It is therefore an object of the present invention to provide a communications system comprising individual modules which are referenced to a single external signal and in which the modules are provided with reference base signals having improved stability and accuracy as well as being subject to less interference and distortion.
It is another object of the present invention to provide a communication system in which the individual modules within the system may automatically switch from internally generated reference signals to a single external signal supplied in common to the modules.
It is a further object of the present invention to provide a communication system having multiple independent modules referenced to a single external signal in which the individual modules can automatically adjust to external reference signals of different frequencies.
It is yet another object of the present invention to provide a communication system having multiple, independent modules which have the capability for automatically switching between reference signals, for adjusting to external reference signals of different frequencies and in which the reference base signals provided to the equipment modules are of improved quality having a frequency spectrum with a reduced level of spurious signals and noise.
It is a yet further object of the present invention to provide a communication system including modules referenced to a common external signal having improved capabilities for processing the external signal and in which the processing circuitry necessary to achieve these ends reflects high standards of design and yet is economical to manufacture.